Gene transcription and its precise regulation underlie development and differentiation, response of an organism to different environmental cues, and many other biological processes. Problems in transcription control are responsible for a number of human diseases, such as cancer. A variety of potential mechanisms for transcription regulation have been discovered in different organisms, and are currently under study. The apparent conservation of some of these mechanisms reflects the fundamental importance of the process. Existence of a well defined multi-protein "Mediator" complex that enables response of the basic transcription machinery to regulatory sequences was first reported in the yeast Saccharomyces cerevisiae. Mediator-like complexes have now been detected in a number of different organisms, and an increasing amount of evidence suggests that they participate in important aspects of transcription regulation throughout all of biology. Structural studies of Mediator complexes will complement their biochemical characterization, and provide invaluable information for the elucidation of their mechanism of action. The work described in this proposal aims to determine the structures of Mediator complexes isolated from a variety of sources, alone, and as part of large molecular complexes involved in transcription. We will obtain comparative information about their overall organization, and study the way in which their structure and behavior are affected by biochemical factors, and by interaction with other components of the transcription apparatus such as RNA polymerase II, general transcription factors, and activators and repressors. By identifying structural differences and similarities between the complexes we hope to contribute to the understanding of what increasingly appears as a universal mechanism for transcription control.